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Aviation History
1974
1974 - 0111.PDF
FLIGHT International, 24 January 1974 103 THE VOYAGE OF MARINER 10 The American spacecraft Mariner 10, the first deep-space probe to be flown on a gravitationally assisted, swing-by flight to two planets, is nearing Venus, its first target. On February 5 it will pass that body at a distance of about 5,000km, taking pictures and measurements while the planet's gravity field changes the speed and trajectory to put it on course for the second target, Mercury, which it will pass at a distance of 1,000km on March 29. Mariner 10 was launched on November 3. The first job was to calibrate and check the instruments, and activation of the science experiments began within 3hr of launch. A series of five test pictures of the Earth and six of the Moon were taken by the two television cameras as the probe began its journey. For the purpose of checking exposure times for picture-taking, the Earth's cloud cover is conveniently similar to that of Venus, while the Moon's rugged surface and lack of atmosphere approximate to conditions on Mercury. TCM-1 (Trajectory Correction Manoeuvre No 1), the first of four course adjustments, was made on November 13, ten days after launch. It was larger than anticipated, at 7-8m/ sec instead of about 4-5m/sec, and brought Mariner 10 nominally back on course from a miss distance of about 65,000km. The second correction, TCM-2, was scheduled for January 18. Extraordinary precision is needed in these so-called course corrections (which are actually the result of velocity changes); a one-mile error from the nominal trajectory at Venus results in the spacecraft being 1,000 miles adrift by the time it reaches Mercury. Accordingly, another pair of corrections will be made after it has rounded Venus and has begun the second stage of its flight. Four problems have arisen, two at least of which are causing some apprehension in the Nasa team at the Jet Propulsion Laboratory, and at Boeing, which built the $98 million spacecraft. They are the refusal of the tele vision-camera heaters to operate when commanded to do so; a partial failure of the plasma-science experiment (designed to measure the intensity of the solar wind nearer the Sun than has previously been possible, and its inter action with the two planets); a power-reset difficulty in the data-handling system; and the apparent loss of the main power-supply system, which caused the spacecraft to switch automatically to the back-up mode. Failure of the camera heaters to operate is thought to be due to the incorrect biassing of a pair of switching tran sistors. Since loss of heat could cause irreparable damage to the television system, the vidicon tubes themselves (the devices which actually form images of whatever is being observed) have had to be kept permanently operating, so that their own power dissipation keeps the temperature from falling too low; Nasa is keeping its fingers crossed that the tubes' lifetimes will see them through to the end of the mission. The cameras themselves have a better resolving power, or ability to create fine images with plenty of detail, than cameras carried aboard any previous American unmanned spacecraft and their performance so far has been excellent. It was planned that pictures of Comet Kohoutek would be taken on January 19. This will be little more than an operational exercise of the cameras, since they are designed to photograph bright landscapes with narrow fields of view, whereas comets need wide- angle, fast cameras to record their relatively faint outlines. This mosaic of the Moon was token by Mariner 10 at a distance of 68,000 miles and shows the remarkable definition achieved b/ its television cameras. Pictures of this quality will be transmitted from a mere 600 miles or so obove Mercury, provided the cameras remain serviceable The main contribution of Mariner 10 to Nasa's wide- ranging Kohoutek programme will be to examine the comet with its extreme-ultra-violet spectrometer, to see whether the tell-tale lines of inert gases (e.g. helium and argon) are present in its spectrum. The difficulty with the plasma-science experiment is thought to be due to the jamming, in a partially open position, of one of the two covers which admit the charged particles to the measuring equipment from forward and aft directions. These covers were designed to be blown off during cruise, their purpose having been to prevent con tamination of the experiment while passing through the atmosphere during launch. The loss of half the experiment should not be too serious, since the distribution of the electrons which form the solar-wind particles is reasonably uniform in all directions. The other sensor should therefore be able to make a comprehensive investigation of any perturbations in the solar wind. The third problem, or anomaly as it is politely termed in Nasa circles, was the automatic resetting of a data- protection system, which occurred when the spacecraft gyros were turned on in preparation for a roll to calibrate the ultra-violet experiment manoeuvre (gyro spin-ups had been accomplished successfully on live previous occasions without the fault showing up). Finally, when some 13-5 million miles from Earth, the spacecraft for some reason not yet understood, switched itself from primary power, generated by the 55 sq ft solar panels, to the back-up supply, a nickel-cadmium battery. Nasa says that there is no way in which primary power can be re-selected, but feels that prospects for a successful flight are still very good, since the chances of a second •failure are quite remote- To conserve power for the encounter with Mercury, Nasa may elect to tut back some of the experiments planned for the Venus fly-by.
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